The age-mass-metallicity-activity relation for solar-type stars: comparisons with asteroseismology and the NGC 188 open cluster (original) (raw)
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Revised ages for stars in the solar neighbourhood
Astronomy & Astrophysics - ASTRON ASTROPHYS, 1998
New ages are computed for the stars from the Edvardsson et al. (1993) data set. The revised values are systematically larger toward older ages (t>4 Gyr), while they are slightly lower for t<4 Gyr. A similar, but considerably smaller trend is present when the ages are computed with the distances based on Hipparcos parallaxes. The resulting age-metallicity relation has a small, but distinct slope of ~ em0.07 dex/Gyr. Tables 3\to8 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or WWW at URL http://cdsweb.u-strasbg.fr/Abstract.html
The age-activity-rotation relationship in solar-type stars
Astronomy & Astrophysics, 2004
We present Ca II K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters to determine the age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr) to the old M 67 (∼4.5 Gyr) through the two intermediate age clusters IC 4651 and NGC 3680 (∼1.7 Gyr). The full amplitude of the activity index within a cluster is slightly above 60 % for all clusters but one, NGC 3680, in which only two stars were observed. As a comparison, the same Solar Ca II index varies by ∼40% during a solar cycle. Four of our clusters (Hyades and Praesepe, IC 4651 and NGC 3680) are pairs of twins as far as age is concerned; the Hyades have the same chromospheric-activity level as Praesepe, at odds with early claims based on X-ray observations. Both stars in NGC 3680 are indistinguishable, as far as chromospheric activity is concerned, from those in the coeval IC 4651. This is a validation of the existence of an age-activity relationship. On the other hand, the two intermediate age clusters have the same activity level as the much older M 67 and the Sun. Our data therefore shows that a dramatic decrease in chromospheric activity takes place in solar stars between the Hyades and the IC 4651 age, of about 1 Gyr. Afterwards, activity remains virtually constant for more than 3 Gyr. We have also measured v sin i for all of our stars and the average rotational velocity shows the same trend as the chromospheric-activity index. We briefly investigate the impact of this result on the age determinations of field G dwarfs in the solar neighborhood; the two main conclusions are that a consistent group of "young" stars (i.e. as active as Hyades stars) is present, and that it is virtually impossible to give accurate chromospheric ages for stars older than ∼2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap.
Stellar mass and age determinations
Astronomy & Astrophysics, 2012
Aims. We present dense grids of stellar models suitable for comparison with observable quantities measured with great precision, such as those derived from binary systems or planet-hosting stars. Methods. We computed new Geneva models without rotation at metallicities Z = 0.006, 0.01, 0.014, 0.02, 0.03 and 0.04 (i.e. [Fe/H] from −0.33 to +0.54) and with mass in small steps from 0.5 to 3.5 M . Great care was taken in the procedure for interpolating between tracks in order to compute isochrones. Results. Several properties of our grids are presented as a function of stellar mass and metallicity. Those include surface properties in the Hertzsprung-Russell diagram, internal properties including mean stellar density, sizes of the convective cores, and global asteroseismic properties. Conclusions. We checked our interpolation procedure and compared interpolated tracks with computed tracks. The deviations are less than 1% in radius and effective temperatures for most of the cases considered. We also checked that the present isochrones provide nice fits to four couples of observed detached binaries and to the observed sequences of the open clusters NGC 3532 and M67. Including atomic diffusion in our models with M < 1.1 M leads to variations in the surface abundances that should be taken into account when comparing with observational data of stars with measured metallicities. For that purpose, iso-Z surf lines are computed. These can be requested for download from a dedicated web page together with tracks at masses and metallicities within the limits covered by the grids. The validity of the relations linking Z and [Fe/H] is also re-assessed in light of the surface abundance variations in low-mass stars.
Astronomy & Astrophysics, 2004
We present homogeneous scales of ages and metallicities for star clusters from very young objects, through intermediate-age ones up to the oldest known clusters. All the selected clusters have integrated spectra in the visible range, as well as reliable determinations of their ages and metallicities. From these spectra equivalent widths (EWs) of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ and Hβ Balmer lines have been measured homogeneously. The analysis of these EWs shows that the EW sums of the metallic and Balmer H lines, separately, are good indicators of cluster age for objects younger than 10 Gyr, and that the former is also sensitive to cluster metallicity for ages greater than 10 Gyr. We propose an iterative procedure for estimating cluster ages by employing two new diagnostic diagrams and age calibrations based on the above EW sums. For clusters older than 10 Gyr, we also provide a calibration to derive their overall metal contents.
Chromospheric activity as age indicator
Astronomy & Astrophysics, 2013
Context. Chromospheric activity has been calibrated and widely used as an age indicator. However, it has been suggested that the viability of this age indicator is, in the best case, limited to stars younger than about 1.5 Gyr. Aims. I aim to define the age range for which chromospheric activity is a robust astrophysical clock. Methods. I collected literature measurements of the S-index in field stars, which is a measure of the strength of the H and K lines of the Ca II and a proxy for chromospheric activity, and exploited the homogeneous database of temperature and age determinations for field stars provided by the Geneva-Copenhagen survey of the solar neighbourhood. Results. Field data, inclusive data previously used to calibrate chromospheric ages, confirm the result found using open cluster data, i.e. there is no decay of chromospheric activity after about 2 Gyr. Conclusions. The only existing indication supporting the viability of chromospheric ages older than 2 Gyr is the similarity of chromospheric activity levels in the components of 35 dwarf binaries. However, even in the most optimistic scenario, uncertainty in age determination for field stars and lack of sufficient data in open clusters make any attempt of calibrating an age activity relationship for old stars premature. The hypothesis that chromospheric activity follows the Skumanich law, i.e. that it is proportional to t −1/2 , should be relaxed.
Astronomy and Astrophysics
A new procedure is described to derive homogeneous relative ages from the Color-Magnitude Diagrams (CMDs) of Galactic globular clusters (GGCs). It is based on the use of a new observable, Delta V(0.05) , namely the difference in magnitude between an arbitrary point on the upper main sequence (V_{+0.05} -the V magnitude of the MS-ridge, 0.05 mag redder than the Main Sequence (MS) Turn-off, (TO)) and the horizontal branch (HB). The observational error associated to Delta V(0.05) is substantially smaller than that of previous age-indicators, keeping the property of being strictly independent of distance and reddening and of being based on theoretical luminosities rather than on still uncertain theoretical temperatures. As an additional bonus, the theoretical models show that Delta V(0.05) has a low dependence on metallicity. Moreover, the estimates of the relative age so obtained are also sufficiently invariant (to within ~ +/- 1 Gyr) with varying adopted models and transformations. Si...
Stellar Chromospheric Activity and Age Relation from Open Clusters in the LAMOST Survey
The Astrophysical Journal
We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al. (2018), the chromospheric activity (CA) indices log R CaK for 1091 member stars in 82 open clusters and log R Hα for 1118 member stars in 83 open clusters are calculated. The relations between the average log R CaK , log R Hα in each open cluster and its age are investigated in different T eff and [Fe/H] ranges. We find that CA starts to decrease slowly from log t = 6.70 to log t = 8.50, and then decreases rapidly until log t = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between log R and log t with 4000K < T eff < 5500K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for log R CaK and 60% for log R Hα .
The Ages of the Hyades, Praesepe, and Coma Star Clusters
Publications of the Astronomical Society of the Pacific, 1969
If one uses the revised T e , (B-V) relations and bolometric corrections for main-sequence stars derived by Morton and Adams (1968) and Iben's evolutionary tracks for Population I stars (X = 0.71, Z = 0.02), it is found that the Hyades and Praesepe clusters are about 9 X 10 8 years old, while-the Coma cluster has an age of about 6.5 X 10 8 years. From a comparison of color magnitude diagrams with evolutionary tracks computed by Auman (1965) and Iben (1965a,b), the Hyades and Praesepe clusters were thought to have an age of between 4 X 10 8 years and 5 X 10 8 years (Barbaro, Dellaporta, and Nobili 1966; Kraft 1967) while the Coma cluster was expected to be somewhat younger since it contains slightly bluer stars. The revised relations between effective temperature (T e), bolometric correction (BC), and color indices of main-sequence stars as derived from interferometric measurements of stellar diameters together with accurate model atmospheres (Morton and Adams 1968), indicate much lower effective temperatures than previously estimated for spectral types between B8 and A5 (c£, Harris 1963); for instance for (B-V) = 0*00, T e has been revised from 10,700° K to 9600° K. This implies considerably larger ages for star clusters in which the turn-off is in this spectral range, since the turn-off is moved farther down the main sequence in the M v , log T e diagram and hence the stars at the turn-off must be less massive than previously estimated. In order to evaluate the magnitude of this effect, timelines were computed for an artificial star cluster that consists of 45 stars with masses between 1.5 39Ω and 2.25 2W©. The 45 mass values were chosen at random from a collection of 180 values that ° Contributions from the Lick Observatory, No. 304.
Age-velocity relations with GALEX FUVFUVFUV-determined ages of Sun-like, solar neighborhood stars
Astrophysics and Space Science
A relationship between chromospheric activity and age is calibrated for FGK dwarf stars using GALEX$FUV$ F U V magnitudes and Gaia$(G_{BP} - G)$ ( G B P − G ) colors. Such a calibration between GALEX$FUV$ F U V magnitudes and stellar age has utility in population studies of dwarfs for further understanding of the chemical evolution of the Milky Way. As an illustration of one such application we have investigated a population of Sun-like, solar neighborhood stars for their metallicities and velocity dispersions; a cross-matched sample of FGK type dwarf stars from Casagrande et al. (2011) with the Gaia and GALEX catalogs. Using calibrated relationships between FUVFUVFUV F U V magnitudes and age, we determined a chromospheric activity indicator, QQQ Q , and stellar age, tau\tau tau τ , for each dwarf. We constructed age-velocity (AVR) and age-metallicity (AMR) relations with empirically-determined FUVFUVFUV F U V ages. Power law fits to AVR plots are consistent with heating mechanism models within...
Age Determination of 15 Old to Intermediate-age Small Magellanic Cloud Star Clusters
We present CMDs in the V and I bands for fifteen star clusters in the Small Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT, Chile). We selected these clusters from our previous work ), wherein we derived cluster radial velocities and metallicities from Calcium II infrared triplet (CaT) spectra also taken with the VLT. We discovered that the ages of six of our clusters have been appreciably underestimated by previous studies, which used comparatively small telescopes, graphically illustrating the need for large apertures to obtain reliable ages of old and intermediate-age SMC star clusters. In particular, three of these clusters, L 4, L 6 and L 110, turn out to be amongst the oldest SMC clusters known, with ages of 7.9 ± 1.1, 8.7 ± 1.2 and 7.6 ± 1.0 Gyr, respectively, helping to fill a possible "SMC cluster age gap" (Glatt et al. 2008a). Using the present ages and metallicities from (Parisi et al. 2009), we analyze the age distribution, age gradient and age metallicity relation (AMR) of a sample of SMC clusters measured homogeneously. There is a suggestion of bimodality in the age distribution but it does not show a constant slope for the first 4 Gyr (Piatti et al. 2011b), and we find no evidence for an age gradient. Due to the improved ages of our cluster sample, we find that our AMR is now better represented in the intermediate/old period than that we derived in Parisi et al. 2009, where we simply took ages available in the literature. Additionally, clusters younger than ∼ 4 Gyr now show better agreement with the bursting model of Pagel & Tautvaišienė (1998), but we confirm that this model is not a good representation of the AMR during the intermediate-age/old period.